Fix definition of gist:Magnitude

[rjyounes]

Definition is now "Base class for units which can be converted. The primitive units can be converted from one measurement system to another; the complex units (ratio or product) have to decompose to their primitives."

Seems to be a copy-paste error. I think we can fix this before the major units and magnitudes refactoring, because unless I'm missing something it's an error.

[uscholdm]

I propose:

skos:definition "A quantity expressed with a unit of measure and a number"^^xsd:string ;
skos:example "3 millimeters; 70 miles per hour; 110 degrees F"^^xsd:string ;

[rjyounes]

Looks good to me.